Low voltage Valve audio amplifiers

[G6Tanuki]

Yes, definitely worth looking at some of the 1930s [before rural 110VAC electrification became a thing] US radios intended for use on 32-Volt ranch-electrics powered by wind- or water-turbines and feeding a shedload of lead-acid batteries.

I remember seeing a schematic for a "Sears Roebuck" radio that used six triodes in push-pull parallel - even so it only managed to produce a Watt or so of audio.

[livewireless123]

I have recently dug out my first car radio, from the late 60s. it's a Philips hybrid set using the ech83 and ebf83 line up. just as an experiment I replaced the the two ech83s with ech81s, it still works as well as before. I had some ebf80s in stock so tried one instead of the ebf83, they have the same basing. again the set works with no discernible difference.
so it looks as though some 'high ht' valves will perform at greatly reduced voltage at low level audio or RF signal levels but not at audio power level.

[kalee20]

I've read that the ECH83 and ECH81 are the same, it's just that the ECH83 is tested under low-voltage conditions. It could even be that the ECH81 is just an ECH83 that didn't quite make the grade.

The capacitances are the same for the two types, which is strong evidence that the electrode configuration is the same (obviously it's not conclusive).

[G6ONEDave]

Although they won't directly drive a loudspeaker, what about the valves that were once used in hearing aids. These valves work from a 22v HT supply and a 1.2v heater voltage. Mullard, Hi-Vac and Ray-o-Vac come to mind. The possible downside is that these valves have wires rather than pins for their connection. Usually they had a 5 way socket that the valve plugged into. You could use them to drive an AD149 or similar if you wanted something older, as the loudspeaker current handling drive.

Dave

[Synchrodyne]

Originally Posted by kalee20

I've read that the ECH83 and ECH81 are the same, it's just that the ECH83 is tested under low-voltage conditions. It could even be that the ECH81 is just an ECH83 that didn't quite make the grade.

The capacitances are the same for the two types, which is strong evidence that the electrode configuration is the same (obviously it's not conclusive).

My understanding is that the ECH81 and ECH83 were essentially the same, but that the low voltage performance of the production might have had quite a bit of scatter, with the ECH83 group being selected on the basis of meeting the specified car radio low voltage performance requirements. Possibly the proportion of valves that met the ECH83 requirements was larger than the number that needed to be labelled ECH83, which might account for the sometimes finding of “no difference” by some who have compared the two (usually with relatively small sample sizes), and so concluded that the renumbering was a marketing not a technical exercise.

A clue is given by Philips Europe own pattern of usage in TV receivers. For a while, it did use the ECH81 as a noise-gated sync separator. Then it switched to the ECH83, presumably because of its better low voltage performance in what was a relatively low HT application. That change would seem to have been an unlikely one absent technical justification. Eventually the purpose-designed ECH84 (with a sharp cutoff heptode) superseded the ECH83 in that role.


Cheers,

[Synchrodyne]

A perhaps unusual use of low voltage car radio type valves was in the Marconi Monitor marine reserve receiver of c.1966.

This was specified to operate on 24- or 220-volts DC, with an AC power supply optional.

Most of the valves were from the car radio series, namely EF97, EF98, ECH83 and EBF83. There was also a D77 (EB91) and the AF output was done by a PL84 pair. I have never found a schematic for this, though.

That the car radio valves were used suggests that the receiver was designed to operate from a 24-volt emergency battery supply without any form of conversion to a higher voltage, possibly to minimize battery current draw.

From that, a reasonable inference is that the PL84 pair was capable of providing some output with 24 volts HT.


Cheers.

[kalee20]

I'll bet they would give some output!

But it's a huge swing from 24V to 220V, I'm amazed that everything works over such a wide range! (Or maybe there was a changeover switch, to re-jig biasing etc?)

[retailer]

I've been sorting and testing my stock of used valves, and the other day I went through a box labelled 6AV6 - in it I found a few that looked like a 6AV6 but were in fact a 12EL6 and 12FK6, as well as a 12EA6 - not sure why it was lumped in with the 6AV6's as it certainly doesn't look like one.
On looking the 3 up I find they are low voltage types designed for valve car radios, max anode voltage 16V

[Diabolical Artificer]

This popped up in "suggested video's on Youtube earlier - https://www.youtube.com/watch?v=HL5B...=UsagiElectric . A futile exercise or a possible solution to the OP's question?

Andy.

[AdrianH]

Just returning to this topic for a while. I have some miniature valves picked up from Birkett's in Lincoln, CV4503 these are wired valves and equiv to EF74s.

I had them wired as triodes with anode and g2 strapped together, but still had g3 strapped to cathode.
With a 13.6 Volt HT, cathode at 0 Volts I used various grid resistors, measured the grid bias and anode current in micro Amps and these were the results I obtained.




I hope they make sense.

Adrian

[daviddeakin]

Here's an article about operating valves at 12V. The main takeaway is that you have to worry about grid current, and the 'normal' way of implementing grid-leak bias is not ideal.
http://valvewizard.co.uk/Triodes_at_...s_Blencowe.pdf

[G.Castle]

If you want to maximise current from the anode of a valve at low voltage supplies you need a
big cathode, preferably run at critical emission.

[Diabolical Artificer]

Found this whilst editing bookmarks, PA amp on D cells? http://diyaudioprojects.com/mirror/m...ch101/#Battery and a 1AA powered amp?

Andy.

[murphyv310]

You will get useful output from a 1ZH29B Russian Rod Pentode at 50v and under, in class B or AB Pretty decent audio. If you can get your hands on a 1ZH42A you could well be in for a surprise at 9v HT, certainly not loud but again on class AB or B enough for reasonable audio in a small room. Even more if you drive a sensitive 18 inch speaker.

[toprepairman]

7233 could be a good contender, 120mA at 50V.